No error message when an unknown option and two args are given

When an option `-u` is not defined:

  * `node foo.js -u` -> unknown option error (expected)
  * `node foo.js -u arg1` -> unknown option error (expected)
  * `node foo.js -u arg1 arg2` -> NO ERROR (unexpected)

Another example: `mocha --reporterrrr spec test` (should be `mocha --reporter spec test`) shows no error.

This bug is caused by `Command.parseArgs()` assuming `arg2` to be a command name.